1. Field of the Invention
The present invention relates to a displacement control valve system provided in a variable displacement compressor for use in automobile air conditioner or the like.
2. Description of the Related Art
Conventionally, a variable displacement compressor has been used in a refrigerating circuit of automobile air conditioner. A displacement control valve system is provided in a rear housing so as to change the volume of cooling refrigerant for compressing this variable displacement compressor. The displacement control valve system includes a valve casing and a solenoid. The valve casing has a pressure sensing space at an end thereof and a valve chamber at the other end. The pressure sensing space is connected to a suction chamber. Inside the sensing space, a bellows portion is disposed inside thereof. A valve chamber communicates with a crank chamber and discharge chamber of the compressor, and a path for communicating therebetween is opened or closed by a valve member accommodated in the valve chamber. An extension/contraction of the bellows portion is converted to a movement for opening/closing the valve via a transmission rod. Further, a solenoid adjusts the opening of this valve member.
In this displacement control valve system, if a cooling load of a compressor increases, an electromagnetic force increases so as to act for reducing a valve travel or valve lift, that is a opening degree of the valve. When the valve travel is decreased, the amount of refrigerant flowing into the crank chamber is decreased. As a result, a pressure of the crank chamber is reduced so that an inclination of the swash plate (angle relative to a plane perpendicular to a driving shaft) increases.
On the other hand, when the cooling load of the compressor is small, the electromagnetic force decreases so as to act for increasing the opening of the valve. As a result, the amount of refrigerant flowing into the crank chamber increases, so that a pressure of the crank chamber increases thereby the inclination of the swash plate being reduced.
This method is called external control method, which enables to change the displacement freely according to an external signal.
In the conventional external control method variable displacement compressor, it has been proposed to enforce the compressor to be maintained at its minimum displacement by detecting vehicle accelerations to reduce power consumption of the compressor, thereby improving the vehicle acceleration performance.
In the conventional displacement control valve system, even if power supply to the solenoid is turned OFF, a force which is a pressure difference acting to close the valve body is left. For example, if a suction chamber pressure exceeds an upper limit for control, the bellows is contracted so that the valve is closed. As a result, no discharge gas is supplied to the crank chamber. Therefore, the displacement cannot be maintained at its minimum level.
Further, such a problem also exists that when a constant current is supplied to the electromagnetic coil of the solenoid, the suction chamber pressure is changed by a discharge chamber pressure thereby a stabilized control being damaged.
Therefore, although a sealing area of the valve body has to be small to reduce an influence of the discharge chamber pressure, the amount of discharge gas introduced to the crank chamber becomes short, so that the displacement control becomes unstable.